Radio frequency (RF) transmitter-receivers and transceivers have been capable of both transmitting and receiving RF signals. Typically, the transmit bands and the receive bands have been offset from each other to minimize interference at the receiver from the outgoing transmitter signals. Other interference reducing techniques, such as the use of special filters such as surface acoustic wave (SAW) filters have also been used to remove unwanted frequency components from the transmitted signals and reduce interference at the receiver.
Many transceivers today use digital predistortion techniques to generate more usable power from amplifiers, avoiding the need for larger amplifiers consuming more power. However, the use of these digital predistortion techniques caused additional transmitter noise to leak into the received signals at the receiver. The additional noise leakage has been reduced by increasing the size of the duplexer to achieve better isolation between transmit and receive bands. However, as wireless devices such as phones, tablets, and other RF devices become smaller and less expensive, these larger and more expensive duplexers have become impractical.
To reduce this additional transmitter noise, a noise cancellation filter covering the entire transmitted signal band was applied to a copy of the signals transmitted at the transmitter in order to estimate the noise from the transmitted signals that was expected to leak into the received signals. As a result, the cancellation filter required setting a large number of taps or filter coefficients that was very expensive to manufacture. The cost and resources required to provide and program this large number of taps make it impractical for many low cost applications. Additionally, each of these taps had to be powered so that as the number of taps increases, the total power consumption by the taps also increased making large numbers of taps impractical for use in low power applications.
Full band noise cancellation filter performance was also degraded by external blockers. External blockers may include signals within a receiver band but not in any active channels. For example, in cellular systems, an external blocker may include another operator's signal on different channels within a given band. Full band noise cancellation filters would consider these additional signals from external sources as noise since they were not included in the transmitted signal and this additional noise had slowed down the rate of convergence of the full band noise cancellation filter due to this additional noise. These external blockers also caused delays and errors during the adaptation process in which filter coefficients were adjusted to improve the noise cancelling capabilities of the full band filter.
In some instances, noise cancellation filters have been designed to be independent of a digital front end of a receiver circuit. This was done because some users prefer to use their own customized digital front end equipment for applying specialized signal processing to the received incoming signals. Digital front end circuits may include a digital down converter, channel specific filters, and other signal processing blocks customized for particular applications.
There is a need for a low power, reduced size RF noise cancellation circuit that is able to cost efficiently minimize transmitter noise leakage in received signals without having its adaptation rate reduced by external blockers. In some instances there is also a need for noise cancellation circuits that are digital front end independent so that customers can apply customized signal processing functions to the digitized received signals.